Conventionally, carbon electrodes containing activated carbon particles are used as polarized electrodes for electric double layer capacitors. The carbon electrodes are generally manufactured by applying electrode materials containing activated carbon particles onto a conductive substrate, and drying the laminate. A currently known method for manufacturing polarized electrodes includes applying onto a conductive substrate electrode materials obtained by dispersing activated carbon particles, conductive particulates, etc. in a dispersion of polytetrafluoroethylene (referred to as “PTFE”, hereinafter) as a binder; and drying the laminate.
The capacitance of an electric double layer capacitor varies depending on the area (effective area) where the activated carbon particles and the conductive particulates contained in the polarized electrodes contact with the electrolyte. More specifically, the capacitance is closely related to the packing ratio of the activated carbon particles and conductive particulates in the polarized electrodes. Therefore, to increase the capacitance, the packing ratio of the activated carbon particles and conductive particulates needs to be improved.
In a slurry method which includes dispersing activated carbon particles, conductive particulates, a binder, etc. in a dispersion medium to prepare slurry-like electrode materials; and forming an electrode layer of the polarized electrode using the same, the capacitance of a polarized electrode is generally proportional to the solid content of the electrode materials. Therefore, to increase the capacitance of the polarized electrode, electrode materials with high solid content need to be prepared in the slurry method.
In general, however, the electrode materials with high solid content cannot exhibit favorable application properties in many cases since such electrode materials are thickened due to a high degree of interaction between the binder and the solid content, such as activated carbon particles, conductive particulates, etc. For example, according to the conventional slurry method, when the solid content is 25% by weight or more, the viscosity of the electrode materials is about 6,000 cP (the viscosity being measured under atmospheric conditions at 25° C. with a Brookfield viscometer). Thus, a high viscosity makes it difficult to demonstrate good application properties.
Therefore, in the slurry method, the solid content of the electrode materials in view of ease of preparation, ease of applying, etc. is preferably adjusted to be about 20% by weight.
In recent years, lightweight and miniaturized polarized electrodes have been demanded, and thus the development of high capacity polarized electrodes for lightweight and miniaturized designs is called for. Thus, it is desired to produce practical polarized electrodes using electrode materials with a solid content of about 30% by weight. The development of electrode materials with low viscosity and excellent application properties (e.g., slurry flowability) even when the solid content thereof is 30% by weight or more needs to be realized.